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Can someone point me at a description of the difference between a 3G-A SDI signal and a 3G-B SDI signal.

I have a rack with a BlackMagic ATEM 2 M/E Production Studio 4K and a bank of 3 SWT Pro-HD monitors. The ATEM is set up to run 1080p59.94. The monitors show "No-sync" when fed from the Aux outputs of the ATEM.

If I insert a Decimator MD-HX between the Aux output and the monitor's SDI input, it shows the SDI input to the MD-HX running 3G-B 1920x1080p59.94. The SDI output of the MD-HX is switchable between 3G-A SDI and 3G-B SDI. Switching he output to 3G-A makes the signal appear on the monitor.

The problem does not appear if I run the ATEM at 720p59.94.

Wikipedia's article on SDI does not mention the A and B variants of SDI.

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Wikipedia does mention it - with a reference link included:

Within this standard there are three formats known as Level A, Level B Dual Link (B-DL) and Level B Dual Stream (B-DS). The Level A format is the direct mapping of uncompressed 1080p (up to 60 fps) video into a serial digital interface at the nominal 3 Gbit/s. The Level B-DL format is the mapping of dual-link HD-SDI/SMPTE 372M (i.e.: 1080p up to 60 fps) in a single serial digital interface at the nominal 3 Gbit/s. The Level B-DS format is the dual-stream carriage of two independent HD-SDI/SMPTE 292M signals (720p up to 60 fps or 1080i/1080p up to 30 fps) in a single serial digital interface at the nominal 3 Gbit/s

The PDF reference has the descriptions starting on pg. 17.

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